U.S. Pat. App. Pub. Nos. 2006/0063054 A1 and 2006/0063055 A1 and U.S. patent application Ser. Nos. 11/261,053, 11/262,268 and 11/962,971, incorporated herein by reference, address the use of cerium or manganese additives to improve durability of polymer electrolyte membranes (PEM's).